This invention relates in general to the field of cellular communications. More specifically, the present invention relates to handing off communications between the cells of a satellite cellular communications system.
A cellular communications system projects any number of cells over the earth at diverse locations. A frequency spectrum is then allocated in frequency, in time, by coding, or a combination of these, to the cells so that communications taking place in nearby cells use different channels to minimize the chances of interference. On the other hand, communications taking place in cells located far apart may use the same channels, and the large distance between communications in common channels prevents interference. Over a large pattern of cells, a frequency spectrum is reused as much as possible by distributing common channels over the entire pattern so that only far apart cells reuse the same spectrum. An efficient use of spectrum results without interference.
One problem which cellular communications systems address is the handing off of communications between cells. Relative movement between subscriber units and cells causes the subscriber units and the communication links directed thereto to move between cells. In order to permit continuous communications in an ongoing call, the system must xe2x80x9chand-offxe2x80x9d the communication when the subscriber unit crosses a cell boundary. If a communication is not handed off to a new cell upon leaving an old cell, the communication will eventually be lost because the strength of signals over which communications take place would diminish to a point where the system""s radio equipment cannot receive the subscriber unit""s transmissions, or vice versa.
Conventional cellular communications systems address the hand-off problem by monitoring and comparing signal strength of nearby cells. A currently used channel associated with one cell may be monitored and compared with channels associated with other cells. This type of monitoring may be performed by a subscriber unit. Alternatively, a currently used channel may be monitored from locations in two different cells, and the results of this monitoring compared. This type of monitoring may be performed by system equipment located in diverse cells. Communications are then passed off to the cell with the stronger signal.
The conventional hand-off technique may work adequately when the distances between subscriber units and system transceivers are relatively small, when the speeds of movement between cells and subscriber units are slow, and when hand-offs are relatively evenly distributed in time. Such conditions are present for conventional terrestrial cellular systems in which cells do not significantly move with respect to the earth and the movement between cells and subscriber units results from subscriber movement in accordance with conventional modes of transportation. On the other hand, when system radio equipment is located on satellites orbiting the earth in moving orbits, these conditions are not present, and the conventional hand-off techniques may be inadequate.
For example, orbiting satellites are located a relatively large distance from subscriber units, often on the order of several hundred kilometers. The smaller this distance, the greater the speed of the satellite relative to a particular position on the earth. Speeds of over 20,000 km/hr are typical. This fast movement relative to a subscriber unit may cause hand-offs to occur much more frequently than in conventional cellular systems. For these types of systems not only does the cell pattern move, but its configuration changes with time. Furthermore, cell shutdown may occur so as to maintain channel separation. This leads to additional hand-off requirements.
In terrestrial cellular systems, subscriber units are directed to remain communicating within a cell as long as possible to prevent bouncing back and forth between cells. The signal level of cells fall off at a slow rate because users generally move relatively slowly in relation individual cells. Because of this relatively slow movement, subscriber units are able to remain communicating within a cell even though other cells may have greater signal levels.
In contrast to terrestrial cellular systems, some satellite based cellular systems have cells that are moving rapidly with respect to the surface of the earth. Because of the rapid movement of cells, hand-off decisions must be made quickly. Furthermore, the inherently long propagation delays in sending and receiving messages in satellite based systems necessitates that hand-off decisions be made early.
Another factor in hand-off decisions is power consumption. Keeping subscriber units within a cell when a better cell is available results in greater power consumption because more signal level is required. Unlike terrestrial cellular communication systems, satellite based systems have limited power resources. Therefore, making hand-off decisions earlier may help reduce power consumption.
Furthermore, satellite based communication systems have less fade margin than terrestrial systems due to the long path lengths between a satellite and a subscriber unit. Because of this reduced fade margin, it is desirable to operate on antenna beams having greater signal levels, and therefore make hand-off decision quickly.
Thus what is needed are a subscriber unit and method that makes hand-off decisions. What is also needed are a subscriber unit and method that makes optimal hand-off decisions, especially in the presence of noise and fading. What is also needed are a subscriber unit and method that makes hand-off decisions quickly. What is also needed are a subscriber unit and method that makes hand-off decisions in a communication system where the cells are moving very rapidly. What is also needed are a subscriber unit and method for handing-off communications that conserves power. What is also needed are a method an apparatus that prevents fading channel measurements from causing an erroneous hand-off.